In a cellular radio system based on time division multiple access (TDMA) or frequency division multiple access (FDMA), the control of transmit power is not necessary regarding the system operation. The first systems thus used the same transmit power all the time. In present cellular radiotelephone systems (e.g. NMT) control of the cellular radiotelephone power is used basically in order to save battery capacity. Power control is further used in small cells in order to reduce the noise level towards adjacent cells. However, usually there are only a few (about 5 to 10) usable power levels.
In a system based on code division multiple access (CDMA), in which the spectrum is spread with the DS technology (Direct Sequence), the power received by the base station from different cellular radiotelephones should be approximately equal, so that the system can accommodate as many users as possible. In the CDMA system the primary object of the power control is thus to maximize the capacity of the system. Then the power level must be controlled very accurately, and the transmit level must be minimized.
The transmit power can be controlled according to two different principles, either as open loop control or as closed loop control. In the open loop control the telephone measures the signal power received from the base station and then controls its transmit power accordingly. Generally the method does not, however, provide a good measurement accuracy, and thus it is neither possible to fully minimize the transmit power, so that it would not be inadvertently controlled too low due to the measurement inaccuracy. The reason for this is that the transmitted and received signals are differently attenuated, because they are situated at different frequencies. The difference can be 10 to 40 dB, depending on the radio signal's bandwidth and the propagation conditions. Thus the transmit power must be dimensioned to be sufficiently high, so that a reliable transmit link can be created. Open loop control is generally used in the present FDD/FDMA and FDD/TDMA systems (FDD, frequency division duplex), in which the transmit and reception is effected at different frequencies. The open loop control method is simple, but its disadvantage is the measurement inaccuracy of the method.
In the closed loop control system the signal power received from the cellular radiotelephone is measured at the base station and the control information required for the control of the transmit power is transmitted to the cellular radiotelephone. The disadvantage of the closed loop control method is that the method is more complicated than the open loop control, because in the closed loop system additional information must be transmitted separately and the transmit of control information in the cellular radio system requires a transmit rate of about 1 kbit/s so that the control system can operate correctly. A control system of this type is proposed to be used in FDD/CDMA systems.